1. Field of the Invention
The present invention relates to an image forming apparatus such as electrophotographic copying machines and printers.
2. Description of the Related Art
An image forming apparatus such as the electrophotographic copying machine has a photoconductive drum which serves as an image carrier, wherein the surface of the photoconductive drum is exposed to form an electrostatic latent image and this electrostatic latent image is then developed by means of developer to form a visible image.
Photoconductive drums of the inorganic type which use arsenic selenium, amorphous silicon (which will be hereinafter referred to as A--Si), and the like, for example, as their photoconductive material are well-known. These drums must be used while being heated to a temperature higher than the ordinary temperature, when their characteristics are taken into consideration. A drum heater or a heating lamp is thus arranged in the drum so as to heat it to a temperature, ranging from 30.degree. C. to 50.degree. C., higher than the ordinary temperature. By heating the drum in this manner, image deterioration due to temperature lowering of the drum can be prevented. In short, image fault such as fog can be prevented in the case of the arsenic selenium drum and image fault such as image flow can be prevented in the case of the A--Si drum.
When a photoconductive drum is used while keeping them heated as described above, however, toner adheres to the drum surface, thereby causing filming and black points. A mechanism causing black points will be described.
The inorganic photoconductive drum such as a arsenic selenium drum, A--Si drum, and the like has appearance faults such as micro-projections and film stripping on its surface and these appearance faults cannot be avoided yet in the course of producing them. When the drum is used while keeping its surface temperature higher than the ordinary temperature or when its temperature is raised depending on a copying mode selected, melting toner adheres to the micro-projections or film-stripped portions on the drum surface. The melted and adhering toner is extended and fixed on the drum surface by the cleaning blade, thereby causing black points.
When an image density of an original to be copied is high, the toner density of a developer image formed on the photoconductive drum becomes high accordingly. The amount of residual toner on the drum surface after the image is transferred to a sheet of paper increases, too. This makes it more likely to cause the above-mentioned image faults such as filming and black points.
In order to prevent these image faults such as filming and black points, it is needed that an auxiliary cleaning mechanism is provided in addition to the cleaning blade to raise cleaning capacity. Mechanisms for applying AC to the photoconductive drum, including cleaning members such as a fur brush and cleaning rollers, and discharging the photoconductive drum by a lamp are well-known as the auxiliary cleaning mechanism.
In the conventional copying machine, however, the cleaning capacity of the auxiliary cleaning mechanism is previously set to meet such a condition that toner is likely to adhere to the surface of the photoconductive drum. Specifically, the operating condition of the auxiliary cleaning mechanism is set to attain a enough cleaning capacity even when image density of the original to be copied is so high as to cause image faults such as filming and black points. Further, the auxiliary cleaning mechanism is always operated under the same condition.
In the auxiliary cleaning mechanism for applying AC, for example, cleaning capacity can be raised by increasing AC voltage applied to the photoconductive drum. Even when an original having a high image density is to be copied, therefore, image faults can be prevented by setting AC voltage high enough. When this high AC voltage is applied to the drum at all times, however, the amount of harmful ozone caused becomes larger. This is not preferable.
In the auxiliary cleaning mechanism provided with a cleaning member such as a fur brush or a cleaning rollers, cleaning efficiency can be raised by increasing the rotation number of the cleaning member. When the rotation number is set high enough, therefore, image faults can be prevented. When the cleaning member is usually operated at this high rotation number, however, its life become shorter and toner adhering to the cleaning member is scattered in the copying machine to a greater extent. This is not preferable, too.
In the auxiliary cleaning mechanism having the discharge lamp, cleaning efficiency can be raised by increasing voltage applied to the lamp to make it brighter. In this case, however, lamp life becomes shorter.